1. Field of the Invention
The present invention relates to a high-strength steel sheet resistant to strength reduction and excellent in low-temperature toughness of a weld heat affected zone (hereinafter, sometimes referred to as “HAZ”), even when subjected for a long time to a stress-relief annealing process (hereinafter, sometimes referred to as “SR process”) after being processed by welding.
2. Description of the Related Art
Makers of large steel pressure vessels (tanks) are promoting on-site assembly of overseas tanks for cost reduction in recent years. It has been usual to complete a tank by carrying out processes including a cutting process for cutting out steel workpieces, a shaping process for bending the steel workpieces, an assembling process for assembling the steel workpieces by welding, an SR process (local heat treatment) for processing some of the steel workpieces, and a final assembling process at the maker's plant and to transport the completed tank to an installation site.
There is a trend, in view of improving efficiency, toward building a tank by carrying out processes for cutting out workpieces, bending the workpieces to produce component members in the maker's plant, transporting the component members, building a tank on site by assembling the component members by welding and processing the entire tank by an on-site SR process.
As the method of building a tank thus changes, time for which the SR process is continued and the number of cycles of the SR process need to be increased from the view point of on-site welding techniques and safety. A fact that the component members of a tank are subjected to an SR process for a time between about 10 to 30 hours in total needs to be taken into consideration in designing materials. It is known that carbide grains contained in a steel agglomerate in large carbide grains remarkably reducing the strength of the steel when the steel is subjected to an SR process for such a long time.
A rolling process combining a controlled rolling process and a controlled cooling process is referred to as TMCP (Thermo-Mechanical Control Process) and is widely used as a process for obtaining a steel having high-strength, high-toughness, and high-weldability while having a low carbon equivalent (hereinafter, the steel is referred to as a “TMCP steel”). The TMCP steels are widely used in growing fields from the steel sheets for welded structures centering on shipbuilding to the steel sheets for pressure vessels such as tanks. Even when a pressure vessel is structured by using such TMCP steel, there is a possibility that the strength of the steel sheet could be remarkably decreased when subjected to an SR process treatment for such a long time.
In order to deal with such situations, the steel sheet is generally made to have high-strength before an SR process; however, in order to maintain high-strength under a severe SR process condition, the steel sheet needs to contain a large amount of an alloy element, which causes a problem that the HAZ toughness (in particular, low-temperature toughness) of welded structures is deteriorated.
As a technique for minimizing strength reduction due to an SR process to the least possible extent, a “tough and hard steel for pressure vessels” containing basically Cr at 0.26 to 0.75% and Mo at 0.45 to 0.60% is presented in, for example, Japanese Patent Application Laid-Open No. S57-116756. In this technique, Cr is added to the steel to suppress the coarsening of carbide grains due to an SR process and to suppress strength reduction due to an SR process. However, the problem that the low-temperature toughness of HAZ is deteriorated because of a large Cr content, remains unsolved in such a steel material.
A “high-strength and tough steel for pressure vessels” containing basically Cr at 0.10 to 1.00% and Mo at 0.45 to 0.60%, is presented in Japanese Patent Application Laid-Open No. S57-120652. The technique intends to suppress the coarsening of Fe3C grains into large M23C6 grains due to processing by a long SR process by adding Cr. In the technique, it is assumed that Cr is contained in a relatively wide range of content; however, only high-strength steels having a Cr content of 0.29% or more are disclosed, and hence it is expected that those high-strength steels are unsatisfactory in the low-temperature toughness of HAZ.